Ink-jet ink, ink set, method for ink-jet printing, ink-jet printing apparatus, ink-jet printing unit and ink cartridge

ABSTRACT

An ink which includes a first pigment, a second pigment, a polymer dispersant and an aqueous medium; wherein the first pigment is a self-dispersing pigment having on its surface at least one anionic group bonded to the surface directly or via an atomic group, the second pigment is dispersed in the aqueous medium by the polymer dispersant, and the polymer dispersant contains benzylmethacrylate as a monomer unit. Diffusion of this ink in the printing medium is suppressed in the cross-sectional direction but not horizontal direction. Thus, the ink can form an ink dot that has a proper dot size, uniform and high density in the dot, with little feathering or blur.

This is a divisional application of application Ser. No. 09/884,094,filed Jun. 20, 2001, now U.S. Pat. No. 6,540,344.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ink-jet ink and an ink-jet printingmethod. It also relates to an ink set, ink cartridge, printing unit andInk-jet printing apparatus. The present invention is applicable for anyapparatus that can print on a printing medium such as printing paper,cloth, leather, OHP paper and nonwoven fabric, especially applicable tobusiness equipments such as printers, copiers and facsimiles utilizingink-jet printing process.

2. Related Background Art

The Ink-jet printing method has various advantages including low noiseoperation, low running cost, high-speed printing, and easy adaptationfor down-sizing and for color printing. Thus, ink-jet printing is widelyemployed for printers, copying machines, or the like. In general, theink employed for printers is selected in consideration of its printingproperties such as ejecting properties and fixing properties and printqualities such as bleeding, optical reflection density, and coloring ofthe printed images.

As is well known, inks are divided into two types according to thecoloring material contained in the ink, i.e., the dye ink and thepigment ink. The pigment ink has various advantages in comparison withthe dye ink, such as superior water resistance and light fastness, andclear character printing.

The pigment in a pigment ink is generally stably dispersed in the ink bythe electric repulsion force of the polymer dispersant which destroysthe agglomeration of the pigment particles due to intermolecular force.Therefore, it is preferable to add a polymer dispersant to an ink in anamount according to the amount of the pigment.

When such a pigment ink is applied onto a recording medium such as plainpaper by an ink-jet printing process for printing characters, thesolvent, e.g. water, of the ink penetrates into the paper and evaporatesinto the ambient air, and the pigment particles agglomerate. In thiscase, the more a polymer dispersant is added, the stronger theagglomeration force of the ink on the paper becomes. Thus, when an Inkdot is formed on a paper sheet with a prescribed volume of the inkejected from an ink-jet head, the diameter of the dot is small and thedot shape remains irregular due to the impact of landing on the paper.Therefore, in order to obtain an ink dot having a sufficient printingdensity and a dot size necessary for forming a printing image free fromwhite stripes or other similar defects, the ejection volume of an inkfrom an ink-jet head must be adjusted rather high. Even with such anadjustment, the presence of a polymer dispersant in a pigment ink, incombination with the penetrability decrease of the ink due to the strongagglomeration force of the pigment particles adsorbed on the dispersant,may delay the fixation of a pigment ink on a printing medium or lowerthe rub-off resistance of the recorded image.

In order to make the dot size larger and to improve the fixationproperties, a penetrant may be added to a pigment ink to enhancepenetration of the ink into a printing medium. However, use of apenetrant may cause undesirable phenomena such as the irregularperipheral shape of the dot (feathering), and penetration of the ink tothe back of the printing medium (back-through), which are undesirablefor the high quality recorded image. Further, since the coloringmaterial penetrates into the printing medium, it often occurs that theoptical density (OD) of the ink dot is not so much increased as the dotsize increases.

To solve such problems, inks containing a self-dispersing pigment havebeen proposed. Such an ink can provide dots of a larger size, since thepigment agglomeration force on the paper is weaker than that inconventional pigment inks in which the pigment is dispersed by adispersant. It, however, is not sufficient yet.

As mentioned above, there is still much to study so as to develop aprinting ink that satisfies various factors that affect the quality ofprinting such as fixability of the ink, enlargement of the ink dot size,uniform density in an ink dot, and high optical density of the inkitself, and further satisfies stability as an ink, especially ejectionstability as an ink-jet ink. The same is said for the printing method.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an ink characterized inthat, a droplet of the ink applied onto the printing medium penetratesappropriately into the printing medium in a direction along the surfaceof the printing medium, but penetrates in a suppressed manner in adirection of the printing medium thickness, to give an ink dot formed onthe printing medium having a proper size, high and uniform image densityand excellent shape almost free from blur or feathering, and besides,the ink does not easily stick to the orifice area of the printing headof the ink-jet printing apparatus if the orifice area is waterrepellent.

Another object of the present invention is to provide an ink-jetprinting method characterized in that an ink droplet applied onto theprinting medium penetrates appropriately into the printing medium in adirection along the surface of the printing medium, but in a suppressedmanner in a direction of the printing medium thickness, to give an inkdot formed on the printing medium having a proper size, high and uniformimage density and excellent shape almost free from blur.

Still another object of the present invention is to provide an ink-jetprinting method that can steadily form a high quality image of highimage density with a reduced amount of ink.

Still another object of the present invention is to provide an ink-jetprinting apparatus that can form a high quality image steadily, and alsoto provide a printing unit and an ink cartridge applicable to it.

Still another object of the present invention is to provide an ink-jetprinting apparatus and an ink set usable for forming an image made fromdots having a proper size, high and uniform image density and excellentshape almost free from blur.

According to one aspect of the present invention, there is provided anink comprising a first pigment, a second pigment, a polymer dispersantand an aqueous medium;

-   -   the first pigment being a self-dispersing pigment having on its        surface at least one anionic group bonded to the surface        directly or via an atomic group,    -   the second pigment being dispersed in the aqueous medium by the        polymer dispersant, and    -   the polymer dispersant containing benzylmethacrylate as a        monomer unit.

According to another aspect of the present invention, there is providedan ink comprising a first pigment, a second pigment, a polymerdispersant and an aqueous medium;

-   -   the first pigment being a self-dispersing pigment having on its        surface at least one anionic group bonded to the surface        directly or via an atomic group,    -   the second pigment being dispersed in the aqueous medium by the        polymer dispersant, and    -   the second pigment has a BET specific surface area not higher        than 300 m²/g, and a DBP oil absorption not higher than 150        ml/100 g.

Both of the above mentioned inks can be used as an ink-jet ink.

According to still another aspect of the present invention, there isprovided an ink-jet printing apparatus comprising an Ink-storing sectioncontaining an ink-jet ink as described above, and an ink-jet head toeject the ink.

According to still another aspect of the present invention, there isprovided a printing unit comprising an ink-storing part containing anink-jet ink as described above, and an ink-jet head to eject the ink.

According to still another aspect of the present invention, there isprovided an ink cartridge comprising an ink-storing part containing anink-jet ink as described above, wherein the cartridge is removableattached to an ink-jet head that ejects the ink, and supplies the ink tothe ink-jet head when attached thereto. Another ink cartridge accordingto another embodiment of the invention comprises an ink-storing partcontaining one of the inks described above.

According to still another aspect of the present invention, there isprovided an ink-jet printing method comprising the steps of:

-   -   ejecting an ink to a printing medium by an ink-jet process, and    -   forming an image on the printing medium,    -   wherein the ink is one of the ink-jet inks described above.

According to still another aspect of the present invention, there isprovided an ink-jet printing method comprising a step of:

-   -   applying a first ink and a second ink on a printing medium in        such a manner that the first and the second inks come in contact        in a liquid state on the printing medium,    -   wherein the first ink contains a self-dispersing pigment having        on its surface at least one anionic group bonded to the surface,        directly or via an atomic group, and    -   the second ink contains an anionic or nonionic polymer        dispersant or both and a pigment which is dispersible with the        aid of the polymer dispersant in an aqueous medium,    -   where the pigment in the second ink has a BET specific surface        area not higher than 300 m²/g, and a DBP oil absorption not        higher than 150 ml/100 g.

According to still another aspect of the present invention, there isprovided an ink-jet printing method comprising a step of:

-   -   applying a first ink and a second ink on a printing medium in        such a manner that the first and the second inks come in contact        in a liquid state on the printing medium,    -   wherein the first ink contains a self-dispersing pigment having        on its surface at least one anionic group bonded to the surface,        directly or via an atomic group, and    -   the second ink contains an anionic or nonionic polymer        dispersant or both and a pigment which is dispersible with the        aid of the polymer dispersant in an aqueous medium,    -   where the polymer dispersant contains benzylmethacrylate as a        monomer unit.

According to still another aspect of the present Invention, there isprovided an ink set comprising a first ink and a second in,

-   -   wherein the first ink contains a self-dispersing pigment having        on its surface at least one anionic group bonded to the surface,        directly or via an atomic group, and    -   the second ink contains an anionic or nonionic polymer        dispersant or both and a pigment which is dispersible with the        aid of the polymer dispersant in an aqueous medium,    -   where the pigment in the second ink has a BET specific surface        area not higher than 300 m²/g, and a DBP oil absorption not        higher than 150 ml/100 g.

According to still another aspect of the present invention, there isprovided an ink set comprising a first ink and a second ink,

-   -   wherein the first ink contains a self-dispersing pigment having        on its surface at least one anionic group bonded to the surface,        directly or via an atomic group, and    -   the second ink contains an anionic or nonionic polymer        dispersant or both and a pigment which is dispersible with the        aid of the polymer dispersant in an aqueous medium.    -   where the polymer dispersant contains benzylmethacrylate as a        monomer unit.

According to still another aspect of the present invention, there isprovided an ink-jet printing apparatus comprising an ink-storing partseparately containing a first ink and a second ink, and an ink-jet headto eject the first ink and the second ink,

-   -   wherein the first ink contains a self-dispersing pigment having        on its surface at least one anionic group bonded to the surface,        directly or via an atomic group, and    -   the second ink contains an anionic or nonionic polymer        dispersant or both and a pigment which is dispersible with the        aid of the polymer dispersant in an aqueous medium,    -   where the pigment in the second ink has a BET specific surface        area not higher than 300 m²/g, and a DBP oil absorption not        higher than 150 ml/100 g.

According to still another aspect of the present invention, there isprovided an ink-jet printing apparatus comprising an ink-storing partseparately containing a first ink and a second ink, and an ink-jet headto eject the first ink and the second ink,

-   -   wherein the first ink contains a self-dispersing pigment having        on its surface at least one anionic group bonded to the surface,        directly or via an atomic group,    -   the second ink contains an anionic or nonionic polymer        dispersant or both and a pigment which is dispersible with the        aid of the polymer dispersant in an aqueous medium, and    -   the polymer dispersant contains benzylmethacrylate as a monomer        unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a head of an ink-jet printingapparatus;

FIG. 2 is a transverse sectional view of a head of an ink-jet printingapparatus;

FIG. 3 is an oblique outside view of a multihead made of the head shownin FIG. 1;

FIG. 4 is an oblique view of a configuration of a printing apparatus;

FIG. 5 is one example of an ink cartridge;

FIG. 6 is a vertical sectional view of one example of ink cartridge;

FIG. 7 is a schematic cross sectional view of another configuration ofan ink-jet printing head;

FIG. 8 is a side view of a schematic configuration of an ink-jetprinting apparatus according to the present invention;

FIG. 9 is a schematic oblique view of an ink-jet printing head having awater-repellent orifice face;

FIG. 10 is a schematic illustration of an ink-jet printing methodaccording to the present invention;

FIG. 11 is a schematic illustration of another ink-jet printing methodaccording to the present invention:

FIG. 12 is a schematic illustration of still another ink-jet printingmethod according to the present invention;

FIG. 13 is a schematic illustration of still another ink-jet printingmethod according to the present invention; and

FIG. 14 is a block diagram showing the control system of the printingapparatus described in FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Studying how to enhance image quality, the inventors have found that anink containing a self-dispersing pigment, a pigment dispersible in anaqueous medium with the aid of a polymer dispersant and the polymerdispersant can satisfy various factors required for image qualityenhancement, and such an ink is very stable. The present invention isbased on that finding.

In such an ink, the dispersion state of the pigments is stablymaintained even when the amount of the polymer dispersant is reduced,presumably because the self-dispersing pigment (the first pigment)serves as a dispersant for the second pigment that is dispersed by thepolymer dispersant. Moreover, when printing is carried out on theprinting medium such as paper with this ink, the dot size is large incomparison with an ink containing the second pigment and the polymerdispersant only or an ink containing the first pigment only, and the dotis uniform, high in optical density, and relatively rapid fixability.

The mechanism of such phenomena is not clear, but presumably is asfollows: The first pigment and the second pigment onto which the polymerdispersant has attached electronically repulse each other in the ink, toweaken the aggregation force between pigments in comparison with in anink containing only the second pigment and the dispersant. When such anink is applied onto paper, diffusion of the second pigment in adirection vertical to the paper surface is difficult because of theattached polymer dispersant, but diffusion in the direction along thepaper surface (in the horizontal direction) is not so difficult incomparison with a conventional ink containing the second pigment and thepolymer dispersant only. When the conventional ink diffuses in thisdirection, the polymer dispersant molecules rapidly intertwine with eachother or cross-link the pigment molecules as the water content decreasesdue to the evaporation and penetration into the printing medium, causingstrong aggregation of pigment. On the other hand, when the ink accordingto the present invention diffuses in this direction, the existence ofthe first pigment prevents or inhibits the macromolecular intertwiningor cross-linking of pigments. In addition, the repulsion between thefirst pigment and the polymer dispersant eases the strong intermolecularforce between pigment molecules. As a result, the ink is liable todiffuse in a direction along the printing medium surface. The diffusionis eased but not in disorder, since it is still under the influence ofpigment aggregation force. This leads to large dot size with a smallerejection amount, dot roundness, and good smoothing effect (smoothness ofthe image formed by plural dots). These phenomena on the paper surfacebecome remarkable when the Ka value by the Bristow method of the ink isless than 1 ml/(m²·msec^(1/2)), that is, when the ink has a relativelylow penetrability to the printing medium, to improve image quality.

As described above, it is predicted that the specific surface area andoil absorption of the second pigment (a pigment dispersible with apolymer dispersant) participates in the ink-jet printing performance ofa pigment ink containing the first (self-dispersing) pigment and thesecond pigment.

For example, when the pigment is carbon black, the higher the specificsurface area and the oil absorption of the carbon black become, thehigher the surface activity of the carbon black becomes, requiring morepolymer dispersant for stable dispersion. However, when the amount ofthe polymer dispersant is increased, the amount of free polymerdispersant also increases in the ink to increase the ink viscosity,which will exert harmful influence on continuous ink ejectionperformance in ink-jet printing.

On the other hand, if the amount of the polymer dispersant is controlledto be in a range satisfying the ejectability, the ink stability thendeteriorates. In general, a pigment having high specific surface areaand high oil absorption is liable to be influenced by pH. Thus, the pHdrop during storage tends to cause problems such as pigment aggregationand precipitation.

Although this phenomenon is not so conspicuous with the ink of theinvention containing the first pigment and the second pigment as withthe ink containing the second pigment only, still it may be problematic.

Thus, it is preferable that the second pigment has a specific surfacearea determined by the BET method (BET specific surface area) not higherthan 300 m²/g and a DBP oil absorption not higher than 150 ml/100 g,when the above problem is considered.

As described above, the ink of the present invention can satisfy variousfactors required for achieving higher image quality at a high level, andalso it is excellent in stability.

Specifically, the ink of the present invention shows high dispersionstability in the ink tank, and can provide printing properties such as alarge dot size, high image density, high rub-off resistance, andexcellent dot roundness.

Although the ink of the present invention described above contains aself-dispersing pigment and a pigment dispersible with a polymerdispersant and the polymer dispersant, the self-dispersing pigment isnot limited to one species but may be two or more pigments, and thepolymer dispersant may be two or more polymer dispersants.

Next, inks of the present invention are explained in detail. The inkaccording to one embodiment of the present invention comprises a firstpigment, for example, a pigment having at least one anionic group bondeddirectly or via another atomic group to the surface of the pigment to beself-dispersing in an aqueous medium, a second pigment dispersible inthe aqueous medium with the aid of a polymer dispersant, and the polymerdispersant.

The ink is described below.

[First Pigment]

The term “self-dispersing pigment” used herein refers to a pigment whichcan be stably dispersed in an aqueous medium such as water, awater-soluble organic solvent and a mixture thereof without using adispersant, not forming pigment aggregates which interfere with thenormal ink ejection from the orifice in the ink-jet printing. As such apigment as described above, those having at least one anionic groupbonded on their surface directly or via an atomic group are suitablyused. The specific examples of such a pigment include carbon blackhaving at least one anionic group bonded to its surface directly or viaan atomic group.

The anionic groups bonded to the surface of the carbon black include,for example, —COOM, —SO₃M, —PO₃HM and —PO₃M₂, —SO₂NH₂, —SO₂NHCOR(wherein M represents a hydrogen atom, alkaline metal, ammonium ororganic ammonium, R represents a linear or branched alkyl group having 1to 12 carbon atoms, a substituted or unsubstituted phenyl group, or asubstituted or unsubstituted naphthyl group). The substituent groups forthe phenyl group or the naphthyl group include, for example, a linear orbranched alkyl group having 1 to 6 carbon atoms.

The alkaline metals represented by “M” described above include, forexample, lithium, sodium and potassium, and the organic ammoniums of “M”include, for example. mono-, di-, and tri-methylammonium, mono-, di-,and tri-ethylammonium and mono-, di-, and tri-methanolammonium.

Of the anionic groups described above, —COOM and —SO₃M are particularlypreferable because a highly stable carbon black dispersion state can beobtained.

It is preferable to use the various anionic groups described above bybonding them to the surface of carbon black via an atomic group. As suchan atomic group, there are, for example, a linear or branched alkylenegroup having 1 to 12 carbon atoms, a substituted or unsubstitutedphenylene group, or a substituted or unsubstituted naphthylene group.The substituent groups for the phenylene group or naphthylene groupinclude, for example, a linear or branched alkyl group having 1 to 6carbon atoms. Specific examples of anionic groups bonded to the surfaceof carbon black via an atomic group include, for example, —C₂H₄COOM,—PhSO₃M and —PhCOOM where Ph represents a phenyl group and M is asdefined above. However, it is to be understood that the presentinvention is not intended to be limited to the specific examples.

Carbon black having the above-described anionic group bonded to itssurface directly or via an atomic group can be produced, for example, asfollows.

A —COONa group can be introduced onto the surface of carbon black, forexample, by subjecting the commercially available carbon black tooxidative treatment with sodium hypochlorite. Alternatively, an—Ar—COONa group (wherein Ar represents an aryl group) may be introducedonto the surface of carbon black by treating an NH₂—Ar—COONa group withnitrous acid, and bonding the formed diazonium salt onto the surface ofcarbon black. However, the present invention is not limited to thesespecific examples.

[Second Pigment]

The second pigment applicable to the ink according to this embodimentincludes those dispersible by an action of a polymer dispersant in adispersion medium of the ink, specifically, in an aqueous medium. Thismeans that such a pigment can be stably dispersed in an aqueous mediumonly when the pigment particles have adsorbed the molecules of a polymerdispersant on the surface. As such pigments, there are, for example,carbon black pigments such as furnace black, lamp black, acetylene blackand channel black. The specific examples of such carbon black pigmentsare described below, and each of the carbon black pigments can be usedsolely or in proper combination.

As described above, carbon black having a BET specific surface area of300 m²/g or less and a DBP oil absorption of 150 ml/100 g or less ispreferably used in the present invention.

Examples of commercially-available carbon black having such propertiesinclude No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52, MA7,MA8 and No. 2200B (all, products of Mitsubishi Chemical IndustriesLimited), RAVEN 1255 (product of Columbian Carbon Japan Limited), REGAL400R, REGAL 330R, REGAL 660R and MOGUL L (all, products of CABOT Co.),and Color Black FW-1, Color Black FW18, Color Black S170, Color BlackS150, Printex 35 and Printex U (all, products of Degussa AG).Alternatively, carbon black newly prepared for the present invention maybe used.

For the first pigment, there is no such limitation on the specificsurface area or oil absorption.

The amount of coloring materials, which is the sum of the first and thesecond pigments, preferably ranges from 0.1 to 15% by weight of the inkweight, more preferably 1 to 10% by weight. The ratio of the firstpigment to the second pigment is preferably in the range of 5/95 to 97/3by weight, more preferably 10/90 to 95/5 by weight. Further morepreferably, the first pigment/the second pigment ratio =9/1 to 4/6. Morepreferably, the amount of the first pigment is larger than that of thesecond pigment. When the amount of the first pigment is larger than thatof the second pigment, the ink obtains not only the dispersionstability, but also the ejection stability including ejection efficiencyand reliability attributable to the less wetting of the surface of theink-ejecting orifice.

[Polymer Dispersant]

As a polymer dispersant for uniformly dispersing the second pigment inan aqueous medium, it is suitable to use that function to disperse thepigment in an aqueous medium when adsorbed on the surface of thepigment.

Specifically, high molecular weight copolymers comprised of unitscontaining an aromatic ring as the hydrophobic component are desirableas the polymer dispersant. Preferable monomer units include styrene,α-methylstyrene, benzylacrylate and benzylmethacrylate, phenylacrylateand phenylmethacrylate. Benzylmethacrylate is especially preferable.When the ink of the present invention was prepared by using a polymerdispersant containing at least benzylmethacrylate monomers for aprinting test, the ink showed desirable wetting property, in this case,the ink was repelled by the orifice area very well. In this case, it isespecially preferable that the orifice area is treated with a siliconink-repellant.

When the first self-dispersing pigment is combined with the secondpigment dispersed by the polymer dispersant at least containingbenzylmethacrylate monomer units are combined, it is considered that thestable dispersion state of the first pigment is effectively maintainedto prevent the ink from attaching to the ink-repellent orifice area,although its mechanism is not clear.

As the hydrophilic component of the polymer dispersant used for pigmentdispersion in water, monomers having carboxyl group are preferably used,for example, acrylic acid, methacrylic acid, maleic acid, itaconic acid,and fumaric acid, but not limited thereto. Acrylic acid and methacrylicacid are preferably used.

The polymer dispersant preferably has an acid value ranging 100-500.

It is especially preferable that the content ratio of the polymerdispersant to the second pigment in the ink is in the range of 10-200%by weight in view of maintaining stable dispersion state of the secondpigment in the Ink and also maintaining the ejection stability of theink.

It is also preferable the weight average molecular weight of the polymerdispersant is in the range of 2000-30000, in view of maintaining stabledispersion state of the second pigment in the ink and also maintainingthe ejection stability of the ink.

Further, it is preferable that the polymer dispersant for the secondpigment is adjusted to a neutral to alkaline pH to enhance thesolubility of the polymer dispersant, and thus to obtain a polymerdispersant of longer shelf life. In this case, however, the pH ispreferably in the range of 7-10, since higher pH may cause corrosion ofvarious members of the ink-jet printing apparatus.

The pH adjusting agent to be used includes organic amines such asmonoethanolamine, diethanolamine and triethanolamine; alkali metalhydroxides such as sodium hydroxide, lithium hydroxide and potassiumhydroxide; organic acids and mineral acids.

[Aqueous Medium]

As an aqueous medium to disperse the first and the second pigments,following water-soluble organic solvents can be used. The water-solubleorganic solvents include, for example, alkyl alcohols having 1 to 5carbon atoms such as methyl alcohol, ethyl alcohol, n-propyl alcohol,isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, tert-butylalcohol, isobutyl alcohol and n-pentanol; amides such asdimethylformamide and dimethylacetamide; ketones or keto alcohols suchas acetone and diacetone alcohol; ethers such as tetrahydrofuran anddioxane; oxyethylene or oxypropylene copolymers such as diethyleneglycol, triethylene glycol, tetraethylene glycol, dipropylene glycol,tripropylene glycol and polyethylene glycol: alkylene glycols of whichthe alkylene group contains 2 to 6 carbon atoms such as ethylene glycol,propylene glycol, trimethylene glycol, triethylene glycol and1,2,6-hexanetriol; glycerol; lower alkyl ethers such as ethylene glycolmonomethyl(or monoethyl) ether, diethylene glycol monomethyl(ormonoethyl) ether and triethylene glycol monomethyl(or monoethyl) ether;lower dialkyl ethers of polyhydric alcohols such as triethylene glycoldimethyl(or diethyl) ether and tetraethylene glycol dimethyl(or diethyl)ether; alkanolamines such as monoethanolamine, diethanolamine andtriethanolamine; sulfolane, N-methyl-2-pyrrolidone, 2-pyrrolidone,1,3-dimethyl-2-imidazolidinone. These water-soluble organic solvents canbe used singly or in combination thereof.

The content of the aqueous organic solvent in the ink may be less than80%, preferably in the range of 5-40%.

[Addition of Dye]

Dyes may be added to the ink of this embodiment. The ink which containsa dye as well as the first and the second pigments as well as adispersant for dispersing the second pigment in an aqueous medium, canform a high quality image free from crazing on a printing medium havinga resin coating layer on the surface. Although the mechanism of crazinginhibition is not clear yet, it is considered that on the printingmedium the pigment is present as fine aggregates and the dye surroundsthe pigment aggregates and fills the interspace of the aggregates. Asalready described, the aggregation force of the second pigment is easedby the presence of the first pigment. In addition, the addition of a dyefurther eases the aggregating force of the second pigment, therebyeffectively suppresses the unevenness in a printed image such as“crazing” which often occurs on a printing medium of less ink absorbencythan the plain paper.

Dyes usable in this embodiment include anionic dyes soluble in anaqueous medium used in this embodiment. For example, known acid dyes,direct dyes and reactive dyes are suitably used. Preferably used aredyes having a disazo- or triazo skeleton structure. Further, preferablytwo or more dyes different in the structure are used together. Dyesother than black dyes, such as cyan, magenta and yellow dyes, may beused as long as the tone of the ink does not change greatly.

[Other Additives]

Besides the above components, a surfactant, an antifoaming agent, anantiseptic and the like may be added to the pigment inks, as needed, toprovide them as inks having desired physical properties.

Examples of the surfactant include anionic surfactants such as fattyacid salts, salts of higher alcohol sulfuric esters, salts of liquidfatty oil sulfuric esters and alkylarylsulfonic acid salts; and nonionicsurfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkylesters, polyoxyethylene sorbitan alkyl esters, acetylene alcohol andacetylene glycol. One or more of these surfactants may be suitablechosen for use.

The amount of the surfactant is desirably within a range of from 0.01 to5% by weight based on the total weight of the ink. It is preferred thatthe amount of the surfactant added be determined in such a manner thatthe surface tension of the resulting ink is at least 30 mN/m (dyne/cm),because the occurrence of deformed printing (inaccurate ink landing) dueto wetting of the orifice can be effectively prevented in the ink-jetprinting system used in the present invention.

Further, it is very effective to make the BET Ka value of the ink nothigher than 1 ml/(m²·msec^(1/2)), in order to further improve the printquality such as sharpness and optical density of the recorded image.

[Printing Method]

The ink of the invention is applied to the printing medium by knownink-applying means, preferably by a known ink-jet printing method whereInk is ejected from a printing head onto the printing medium. Theink-ejection method suitable for the ink of the invention can be anyknown methods such as the piezo ink-jet method. However, a preferablemethod is such that a bubble is generated in the ink utilizing heatenergy and the ink is ejected by the pressure of the bubble.

The ink amount applied to the printing medium from the printing head ispreferable not higher than 0.014 picoliter (pl) per unit area. Morespecifically, the preferable ink shot is 70 pl or less at 360 dpi, 25 plor less at 600 dpi. The reason is as follows: When an ink containing onepigment as a coloring material is used, the area factor is insufficient,especially on a normal paper, resulting in low OD. On the other hand,when an ink of the present invention is used, as described above, thearea factor becomes so high that the application amount can be reduced.

The present invention also provides ink containers such as an inkcartridge, and a printing head in which an ink-storing part and meansfor ejecting the ink are integrated.

Further, the present invention provides an ink set comprised of theblack ink of the invention and at least one color ink selected from, forexample, yellow ink, magenta ink and cyan ink, each of which containedin a separate ink-storing part.

The effect of the present invention described above that the interactionbetween the first pigment and the second pigment and the polymerdispersant eases the pigment aggregation force thus leading to excellentimage formation on the printing medium is not only achieved by using anink containing these components but also by contacting these componentseach other in a liquid state on the printing medium. More specifically,the latter method is explained referring to FIG. 10: a first inkcontaining the first pigment is ejected from the first ink-jet head 1001to a predetermined position of the printing medium, and a second inkcontaining a polymer dispersant being nonionic or having the samepolarity to the group attached to the first pigment and the secondpigment is ejected from the second ink-jet head 1002 to thepredetermined position where the first ink was applied or to be applied,in such a manner that the first and the second inks are mixed in aliquid state on the printing medium. This method also forms an ink dothaving a large area factor and excellent shape, and high and uniformimage density in the dot. It is needless to say that when two inks areseparately applied to the printing medium, complete superposition of thelanding positions is not required so long as these inks contact eachother in a liquid state on the printing medium.

Alternatively, as illustrated in FIG. 11, a first ink (1104) containingthe first pigment is ejected from the first ink-jet head 1101, a secondink (1105) containing the second pigment and a polymer dispersant beingnonionic or having the same polarity as the group bonded to the firstpigment is ejected from the second ink-jet head 1102, and these inkdroplets become one droplet (1106) before landing and then the droplet1106 is applied to a predetermined position of the printing medium 1103.This method too can achieve mixing of the two inks in a liquid state onthe printing medium. It is preferable for a mixture of the first andsecond inks to have a Ka of 1 ml/m²·msec^(1/2) determined by the Bristowmethod by adjusting properties of both inks.

Further, the effect of the present invention described above that theinteraction between the first pigment and the second pigment, thepolymer dispersant and a dye eases the pigment aggregation force thusleading to excellent image formation on the printing medium having acoating layer is not only achieved by using an ink containing thesecomponents but also by contacting these components each other in aliquid state on the printing medium. More specifically, the lattermethod is explained referring to FIG. 12: a first ink containing thefirst pigment is ejected from the first ink-jet head 1201 to apredetermined position of the printing medium 1204, a second inkcontaining a second pigment and a polymer dispersant being nonionic orhaving the same polarity to the group attached to the first pigment isejected from the second ink-jet had 1202 to the predetermined position,and a third ink containing a dye having the same polarity as the groupbonded to the first pigment is ejected from the third ink-jet head 1203to the predetermined position, in such a manner that the first to thirdinks are mixed in a liquid state on the printing medium. This methodalso forms an ink dot having a large area factor and excellent shape,and high and uniform image density in the dot. It is needless to saythat when two inks are separately applied to the printing medium,complete superpose of the landing positions is not required so long asthese inks contact each other in a liquid state on the printing medium.

Alternatively, as illustrated in FIG. 13, a first ink (1302) containingthe first pigment is ejected from the first ink-jet head 1301, a secondink (1304) containing the second pigment and a polymer dispersant beingnonionic or having the same polarity as the group bonded to the firstpigment is ejected from the second ink-jet head 1303, and the third ink(1306) containing a dye having the same polarity as the group attachedto the first pigment is ejected from the third ink-jet head 1305, andthese ink droplets become one droplet (1307) before landing and then thedroplet 1307 is applied to a predetermined position of the printingmedium 1308. This method too can achieve mixing of the three inks in aliquid state on the printing medium. It is preferable for a mixture ofthe first to third inks to have a Ka of 1 ml/m²·msec^(1/2) determined bythe Bristow method by adjusting properties of both inks.

[Printing Medium]

The printing medium used in the present invention includes paper,nonwoven fabric, OHP paper and leather, but not limited thereto. Asdescribed above, when the ink of the present Invention is applied by theink-jet method onto a printing medium having a resin layer as a coatinglayer, the “crazing” of the image formed on the coating layer iseffectively prevented.

For example, there are printing media having a resin layer provided on aplastic film or paper. The main component of the coating layer is awater-soluble resin or a resin dispersible in water, which may containother additives such as a cationic compound, a surfactant and a filler,if necessary.

The water-soluble resin includes, for example, polyvinyl alcohol andmodified polyvinyl alcohols such as anion-modified polyvinyl alcohol andacetal-modified polyvinyl alcohol; water-based polyurethane: polyvinylpyrrolidone and modified polyvinyl pyrrolidones such as copolymer ofvinyl pyrrolidone and vinyl acetate, copolymer of vinyl pyrrolidone anddimethylaminoethyl methacrylate, copolymer of quaternaryvinylpyrrolidone and dimethylaminoethyl methacrylate, and copolymer ofvinylpyrrolidone and methacrylamidopropyl trimethylammonium chloride;modified cellulose such as cationized hydroxyethyl cellulose andwater-soluble cellulose resins such as carboxymethyl cellulose,hydroxyethyl cellulose and hydroxypropyl cellulose; synthetic resinssuch as polyester, polyacrylic acid (ester), melamine resin, or modifiedresins thereof, graft copolymer containing at least polyester andpolyurethane; and natural resins such as albumin, gelatin, casein,starch, cationized starch, gum arabic, and sodium alginate. Thewater-dispersing resin includes, for example, polyvinyl acetate,ethylene-vinyl acetate copolymer, polystyrene, styrene-(meth)acrylateester copolymer, (meth)acrylate ester polymer, copolymer of vinylacetate and (meth)acrylate (ester), poly(meth)acrylamide,meth(acrylamide copolymer, styrene-isoprene copolymer, styrene-butadienecopolymer, polyvinyl ether, and silicon-acrylic copolymer, but notlimited thereto.

Alternatively, a cationic compound may be preferably used as thematerial of the coating layer of the printing medium. Any cationiccompound may be used so long as it has a cationic portion in themolecule, for example, there included are cationic surfactants ofquaternary ammonium salt such as monoalkylammonium chloride,dialkylammonium chloride, tetramethylammonium chloride,trimethylphenylammonium chloride, and ethylene oxide adduct of ammoniumchloride; cationic surfactants of amine salt; amphoteric surfactantshaving a cationic portion such as alkyl betaine, imidazolium betaine andalanine; polymer or oligomer such as cation-modified polyacrylamide,copolymer of acrylamide and a cationic monomer, polyallylamine,polyamine sulfone, polyvinyl amine polyethylene amine, polyethylenemine, polyamide-epichlorhydrin resin, and polyvinylpyridinium halide.

Further, vinyloxazolidone polymer or copolymer with other commonmonomer, and vinyl imidazole polymer or copolymer with other commonmonomer can be used.

Monomers usable for the above copolymers include methacrylate, acrylate,acrylonitrile, vinyl ether, vinyl acetate, ethylene and styrene.Cation-modified cellulose can be also used.

Preferable cationic compounds are as listed above, but not limitedthereto.

Concerning the layer thickness, it is preferable to form a coating layerof 0.1 g/m to 100 g/m² by dry weight. preferable. The coating layer maybe single-layered or multi-layered such as two-layered or three-layered.

When the ink of the present invention is applied onto a printing mediumhaving a coating layer as described above, “crazing” can be prevented.In addition, the high wettability of the pigment to the coating layercan compensate the poor wettability of a dye. That is, by using thepigment ink of the invention and a dye ink in mixture, the beadingphenomenon which may occur with single use of the dye ink can besuppressed.

First, a configuration example of the main part, the head, of an ink-jetprinting apparatus utilizing thermal energy is shown in FIGS. 1 and 2.

FIG. 1 is a sectional view of a head 13 along the ink flow path and FIG.2 is a sectional view taken on line 2—2 of FIG. 1. The head 13 isobtained by adhesion of a heat generating head 15 used for thethermosensitive printing to a glass, ceramic or plastic plate having agroove 14 through which ink flows. The heat generating head 15 comprisesa protective film 16 formed of silicon oxide, silicon nitride, siliconcarbide or the like, electrodes 17-1 and 17-2 made of aluminum, gold,aluminum-copper alloy etc., a heat generating resistance layer 18 formedof a high melting point substance such as HfB2, TaN and TaAl, a heataccumulating layer 19 made of aluminum oxide, and a highlyheat-radiating substrate 20 made of silicon, aluminum and aluminumnitride.

When an electric signal information item is applied to the aluminumelectrodes 17-1 and 17-2, the region designated with n of the heatgenerating head 15 is rapidly heated, a bubble is generated in the inkcontacting here, the meniscus 23 protrudes under this pressure and theink is ejected to make an ink droplet 24 and fly to a printing medium 25from the discharge orifice 22.

FIG. 3 shows an appearance view of a multihead comprised of an array ofmany heads shown in FIG. 1. The relevant multihead is fabricated by aclose adhesion of a heat generating head 28 similar to that described inFIG. 1 to a glass plate 27 having a multi-groove 26. Other than themultihead using glass plate shown above, multiheads using resinmaterials can also be used in the present invention. If a multihead ofresin material is used, the head preferably is water repellent toprevent ink from attaching to the orifice area. A water-repellant layer901 containing silicon material or fluorine material onto the orificearea can provide the orifice area with water-repellency. Such a layercan be formed by applying a water repelling agent containing a siliconmaterial or fluorine material.

FIG. 4 shows one example of an ink-jet printing apparatus with the abovehead incorporated. In FIG. 4, numeral 61 denotes a blade serving as thewiping member, one end of which is retained by a blade retaining memberto make a stationary end, thereby shaping a cantilever as a whole. Theblade 61 is disposed at a position adjacent to the printing area by theprinting head 65 and is retained in the shape of protruding into themoving route of the printing head 65 in case of this example. Numeral 62denotes a cap on the ejection orifice surface of the printing head 65,which is located at the home position adjacent to the blade 61 and soarranged as to move in a direction perpendicular to the movement of theprinting head 65, to butt against the ink ejection orifice and tofulfill the capping. Furthermore, numeral 63 denotes an ink absorberprovided adjacently to the blade 61, which is retained in the shape ofprotruding into the moving route of the printing head 65 as with theblade 61.

A discharge recovering section 64 comprises the above blade 61, theabove cap 62 and the above ink absorber 63 and the moisture, dust andthe like on the ink ejection orifice are removed by the blade 61 and theink absorber 63.

Numerals 65 and 66 denotes a printing head equipped with dischargeenergy generating means which discharges ink to the printing mediumopposed to the ejection orifice face from a ejection orifice disposed toperform printing and an carriage for loading and moving the printinghead 65, respectively. The carriage 66 is slidably engaged with a guideshaft 67 and part of the carriage 66 is connected (unillustrated) to thebelt 69 driven by a motor 68. Thereby, the carriage 66 is enabled tomove along the guide shaft 67, thus enabling the printing area by theprinting head 65 and its adjacent areas to move.

Numerals 51 and 52 denote a paper feed section for inserting a printingmedium and a paper feed roller driven by an unillustrated motor,respectively.

By these constituents, the printing medium is fed to the positionopposed to the ejection orifice face of the printing head 65 and paperis discharged to the paper discharge section disposed with the paperdischarge roller 53 according to the progress of printing.

In a return of the printing head 65 to the home position at the end ofprinting or the like with the above configuration, the cap 62 of thedischarge recovering section 64 is averted from the moving route of theprinting head 65, whereas the blade 61 protrudes into the moving route.As a result, the ejection orifice face of the printing head 65 is wiped.When the cap 62 caps the ejection orifice face of the printing head 65by butting, the cap 62 moves in such a manner as to protrude into themoving route of the printing head.

When the printing head 65 moves from the home position to the printingstart position, the cap 62 and the blade 61 stand at the same positionas that of the above wiping. As a result, the ejection orifice face ofthe printing head 65 is wiped also in this move.

The above move of the printing head 65 to the home position is not onlyperformed at the completion of printing and at the recovery ofdischarge, but also the printing head 65 moves at given Intervals to thehome position adjacent to the printing area while moving through theprinting area for printing and the above wiping is carried out with thismove.

FIG. 5 shows one example of an ink supply member for the head, an inkcartridge 45 for storing the ink supplied via a tube. Here, numeral 40denotes an ink-storing section for storing the supply ink, e.g. an inkbag, at the tip of which a rubber stopper 42 is provided. By inserting aneedle (unillustrated) into this stopper 42, the ink in the ink sack 40is enabled to be supplied to the head. Numeral 44 denotes an inkabsorber for receiving the waste ink. As the ink store section, it ispreferable for the present invention that the liquid contact surfacewith ink is made of polyolefin, especially polyethylene.

An ink-jet printing apparatus according to the present invention is notlimited to those comprising a head and an ink cartridge separately asmentioned above, but is also appropriately applied to integrated one asshown in FIG. 6. In FIG. 6, numeral 70 denotes a printing unit in whichan ink-storing section for storing ink, e.g. an ink absorber, isaccommodated and the ink contained in the ink absorber is discharged asan ink droplet from the head section 71 having a plurality of orifices.As materials of an ink absorber, polyurethane is preferably used.

Next, as another embodiment of an ink-jet printing apparatus making gooduse of mechanical energy, is described an On-Demand type ink-jetprinting apparatus comprising a nozzle-forming substrate having aplurality of nozzles, pressure-generating devices composed of apiezoelectric material and an electric conductive material provided inan opposing relation to the nozzles, and an ink filled around thepressure-generating devices, wherein the pressure-generating devices arechanged by voltage applied to eject droplets of the ink from thenozzles. An example of the construction of a printing head, which is amain component of such a printing apparatus, is illustrated in FIG. 7.

The head is composed of an ink flow path 80 communicating with an inkchamber (not illustrated), an orifice plate 81 through which inkdroplets having a desired volume are ejected, a vibration plate 82 fordirectly applying a pressure to the ink, a piezoelectric element 83bonded to the vibration plate 82 undergoing a change according to anelectric signal, and a substrate 84 adapted to support and fix theorifice plate 81, the vibration plate 82 and the like thereon.

In FIG. 7, the ink flow path 80 is formed with a photosensitive resin orthe like. The orifice plate 81 is made of a metal such as stainlesssteel or nickel, and an ejection opening 85 which is formed byelectroforming, punching by press-working, or the like. The vibrationplate 82 is formed with a film of a metal such as stainless steel,nickel or titanium and a high-modulus resin film or the like. Thepiezoelectric element 83 is made of a dielectric material such as bariumtitanate or PZT (lead zircoate titanate; pb[ZrTi]O₂).

The printing head with the above construction is operated in such amanner that pulsed voltage is applied to the piezoelectric element 83 togenerate a stress to cause strain, the vibration plate 82 bonded to thepiezoelectric element 83 is deformed by the energy of the stress, andthe ink in the ink flow path 80 is thus perpendicularly pressurized toeject Ink droplets (not illustrated) from the ejection opening 85 of theorifice plate 81, thereby conducting printing.

Such a printing head is used by incorporating it into an ink-jetprinting apparatus similar to that illustrated in FIG. 4. Operation ofdetails of the ink-jet printing apparatus may be conducted in the samemanner as described above.

As described above, according to the present invention, an inkcontaining a self-dispersing pigment, a pigment dispersible in anaqueous medium with the aid of a polymer dispersant and the polymerdispersant has excellent storage stability, since the dispersion stateof the pigments is stably maintained even when the amount of the polymerdispersant is reduced, presumably because the self-dispersing pigment(the first pigment) serves as a dispersant for the second pigment thatis dispersed by the polymer dispersant. On the other hand, when printingis carried out on paper with this ink, it gives ink dots of properwidth, of uniform image density and almost free from blur, with the fineparticles of pigment aggregates distributing uniformly in the dots,probably because the pigment aggregation due to the interaction betweenthe second pigment and the polymer dispersant on the printing medium iseased by the first pigment. In addition, according to the presentinvention, sticking of the ink to the water repellent orifice area canbe very effectively prevented for carrying out stable ink-jet printing.

Further, an ink of the present invention has a stable poor wettabilityto the orifice area. Here, “poor wettability” means the ink hardlyattaches to the orifice area or easily removed even if it attaches toit. Thus, the accuracy of the ink election direction becomes steady,which is very effective in stable formation of high grade images.Further, with another ink of the present invention which contains a dyein addition to the first pigment, the second pigment and the polymerdispersant, the pigment aggregates take a fine particulate form on theprinting medium probably because the addition of a dye further eases theaggregating force of the second pigment, and the dye surrounds theparticulate aggregates. Unevenness of the printed image is suppressed asa whole. This effect is remarkably observed as the suppression orprevention of crazing when the ink-jet printing is carried out on aprinting medium of poor ink absorbency.

EXAMPLES

Examples according to the above embodiments are described.

Example 1

FIG. 8 is a side view schematically illustrating a full line typeprinting apparatus according to the first example of the presentinvention.

This printing apparatus 1 adopts an ink-jet printing method in whichprinting is carried out by ejecting an ink from multiple full line typeprint heads (ejecting portions) arranged in prescribed positions towardthe direction in which a printing medium as a printing medium isconveyed (in the direction shown by the arrow A in the figure). Itoperates under control of a control circuit (not shown).

Each print head 101Bk, 101C, 101M and 101Y of a head group 101 g hasabout 7200 ink-ejecting ports arranged in the transverse direction of aprinting paper 103 (in the direction perpendicular to the sheet shown inthe figure) conveyed in the direction shown by the arrow A in thefigure. The printing apparatus can perform printing on a printing paperto a maximum size of A3.

The printing paper 103 is conveyed in the direction A with the aid ofrotation of a pair of resist rollers 114 which is driven by a conveyancemotor, guided by a pair of guide plates 115 so as to register its tips,and conveyed by a conveyance belt 111. The conveyance belt 111 which isan endless belt is held by two rollers 112 and 113, and the displacementin vertical direction of its upper side portion is regulated by a platen104. When the roller 113 is rotationally driven, the printing paper isconveyed. The printing paper 113 is held to the conveyance belt 111 byelectrostatic holding. The roller 113 is rotationally driven by adriving source, such as a motor, not shown in the figure in such adirection that the printing paper 103 is conveyed in the direction shownby the arrow A. The printing paper 103 having been subjected to printingwhile being conveyed on the conveyance belt 111 is delivered onto astocker 116.

Print heads of a printing head group 101 g consisting of a head 101Bkfor ejecting a black ink, and heads for ejecting color inks (a cyan head101C, a magenta head 101M and an yellow head 101Y) are arranged in thedirection A in which the printing paper 103 is conveyed. Print of blackletters and color images are made by ejecting each inks from each printhead.

FIG. 14 is a block diagram illustrating a control system of the fullline type printing apparatus 1 of FIG. 8.

A system controller 201 includes a microprocessor, ROM for storing acontrol program executed in this apparatus and RAM used as a work areawhen the microprocessor performs processing, and controls the entireapparatus. A motor 204 rotates the roller 113 shown in FIG. 6 to conveyprinting paper while its drive is being controlled via a driver 202.

A host computer 206 transfers the information to be printed to theprinting apparatus 1 of this embodiment and controls the printingoperation. A receiving buffer 207 temporarily stores the data from thehost computer 206 until the system controller 201 reads the data. Aframe memory 208 is a memory for expanding the data to be printed toimage data and has a memory size required for printing. Although theframe memory 208 is described as being capable of storing data for asheet of printing paper in this embodiment, the present invention is notintended to be limited to a specific memory capacity.

Buffer 209P temporarily stores the data to be printed, and their storagecapacity varies depending on the number of ejecting ports of printheads. A printing controller 210 is provided for properly controllingthe drive of print heads under the command from the system controller201 and controls drive frequency, the number of printing data, etc. Adriver 211 drives the print heads 101Bk, 101C, 101M and 101Y forejecting their respective inks and is controlled by the signal from theprinting controller 210.

In the above system, print data are transferred from the host computer206 to the receiving buffer 207 to be temporarily stored. The print datastored in the receiving buffer 207 is read by the system controller 201and expanded to the buffers 209S and 209P. And paper jam, out-of-ink,out-of-paper, etc. can be detected by various detection signals from atrouble sensor 222.

The printing controller 210 controls the ejection operation of eachprint head based on the print data in the buffer and 209P and the liquidtreatment data.

In this example, ink-ejecting ports of each print head were arranged ata density of 600 dpi, and printing was carried out at a dot density of600 dpi in a direction of the conveyance of the printing paper.Accordingly, the dot density of a printed image etc. in this example is600 dpi in both row and column. The ejecting frequency of each head was4 KHz, and each head ejects 15 pl per ejection.

In the ink-jet printing apparatus in this Example, the distance Dibetween the black head 101Bk and the cyan head 101C is relatively wideas shown in FIG. 8, which can inhibit the color mixing due to featheringon the boundary between a region printed with a black ink and a regionprinted with a color ink. However, if a print medium having a coatinglayer is exclusively used, as feathering itself is inhibited, Di can bemade shorter which enables making the apparatus size smaller.

The composition of the black ink used in this Example is shown below. InTables shown below, total of the components makes 100 parts by weight,and Acetylenol EH is a product of Kawaken Fine Chemicals Co., Ltd.

TABLE 1 Pigment dispersant 1 45 parts  Pigment dispersant 4 5 partsGlycerol 6 parts Diethylene glycol 5 parts Acetylenol EH 0.1 part   Water balance

The pigment dispersions were prepared as follows: 300 g of acid carbonblack (trade name: MA-77, pH 3.0, a product of Mitsubishi ChemicalIndustries, Ltd.) was mixed with 100 ml water, to which 450 g of sodiumhypochlorite (active chlorine concentration: 12%) was added dropwise,and stirred for 10 hours at 100-105° C. Thus obtained slurry wasfiltered with TOYO filter paper No. 2 (from Advantist Co., Ltd.), andthe pigment particles were fully water-washed, then the pigment wet cakewas resuspended in water to deionize to a conductivity 0.2 μS by using areverse osmotic membrane. Then the pigment dispersion (pH 8-10) wasconcentrated to a pigment concentration of 10%. Thus, obtained was apigment dispersion in which an anionically charged self-dispersingcarbon black having —COOH group directly bonded onto the surface wasdispersed.

The pigment dispersion 4 was prepared as follows: 5 parts ofbenzylmethacrylate-methacrylate copolymer (weight ratio 1:1, acid value400, average molecular weight of 9000), as a dispersant, 4 parts ofmonoethanolamine and 81 parts of water were mixed. And the mixture washeated to 70° C. in a water bath to completely dissolve the resincontent. Resin cannot sometimes be completely dissolved if its contentis low. Accordingly, when intending to dissolve resin, a solution with adesired resin content may be prepared by diluting the solution with ahigh resin content previously prepared. To this solution added was 10parts of carbon black of surface area by BET method 150 m²/g, and of DBPoil absorbency 50 ml/g that is not dispersible in an aqueous solutionwithout a dispersant, and the solution was subjected to premixing for 30minutes. Then the following operation was carried out to obtain thepigment dispersion 2 in which the carbon black was dispersed in anaqueous medium with the aid of a dispersant.

Disperser: Side Grinder

-   -   (Igarashi Machine Industry Co., Ltd.)

Grinding medium: zirconia beads with a 1 mm diameter

Packing of grinding medium: 50% (by volume)

Grinding duration: 3 hours

Centrifuging (12000 RPM, 20 minutes)

Thus obtained ink has a Ka value of 0.32 ml/m²·msec^(1/2).

The ink prepared as above was filled in an ink tank of an ink-jetprinting apparatus described above, and printing was carried out at anejection frequency of 7.2 kHz. The resolution of the printing apparatuswas 360 dpi×720 dpi, and the orifice area had been treated with asilicon ink-repellent As a result, obtained was a good printed imagehaving large dot size, high optical density, high rub-off resistance,high dot roundness, and no white streaks.

The ink was repelled by the orifice area very well. Thus, high ejectionperformance was sustained. After 1×10⁸ pulses, ejection clogging orirregular ejection was not observed.

Example 2

Another Bk ink may have the following composition.

TABLE 2 Pigment dispersion 2 40 parts Pigment dispersion 4 10 partsGlycerol  6 parts Acetylenol EH 0.2% Water balance

Pigment dispersion 2 was prepared as follows: 10 g of carbon black witha surface area of 230 m²/g by BET and a DBP oil absorption of 70 ml/100g and 3.41 g of p-aminobenzoic acid were fully mixed in 72 g of water,then 1.62 g of nitric acid was added dropwise and stirred at 70° C.After several minutes, a solution of 1.07 g of sodium nitrite in 5 g ofwater was added and stirred for an additional 1 hour. The slurryobtained in this manner was filtered with Toyo filter paper No. 2 (fromAdvantist Co., Ltd.), and its pigment particles were fully water-washed,followed by the drying in an oven at 90° C. Then water was added to thepigment to prepare 10 wt % pigment aqueous solution. Thus, obtained waspigment dispersion 2 in which an anionically charged self-dispersingcarbon black having a hydrophilic group bonded onto its surface via aphenyl group, as shown by the following formula, was dispersed.

The pigment dispersion 4 was prepared in the same manner as in Example1.

Thus obtained ink has a Ka value of 0.4 (ml/m²·msec^(1/2)).

The ink prepared as above was filled in an ink tank of an ink-jetprinting apparatus described above, and printing was carried out at anejection frequency of 7.2 kHz. The resolution of the printing apparatuswas 360 dpi×720 dpi, and the orifice area had been treated with asilicon ink-repellent. As a result, obtained was a good printed imagehaving large dot size, high optical density, high rub-off resistance,high dot roundness, and no white streaks.

The ink was repelled by the orifice area very well. Thus, high ejectionperformance was sustained. After 1×10⁸ pulses, ejection clogging orirregular ejection was not observed.

Another Bk ink may have the following composition.

TABLE 3 Pigment dispersion 3 47 parts  Pigment dispersion 4 3 parts C.I.Direct Blue 199 1 part  Glycerol 6 parts Diethylene glycol 5 partsAcetylenol EH 0.1% Water balance

The pigment dispersion 3 was prepared as follows:

A solution was prepared by dissolving 5 g of conc. HCl 5 into 5.3 g ofwater, and into which 1.85 g of anthranilic acid was added at 5° C. Bymaintaining the solution of anthranilic acid at 10° C. or lower, asolution of 8.7 g of sodium nitrite in 8.5 g of water (5° C.) was addedand stirred for 15 minutes. To this solution, 20 g of carbon black witha surface area of 320 m²/g by BET and a DBP oil absorption of 120 ml/100g was added and stirred for 15 minutes. The slurry obtained in thismanner was filtered with TOYO filter paper No. 2 (from Advantist Co.,Ltd.), and the pigment particles were fully washed with water, and thendried in an oven at 110° C. Then water was added to the pigment toprepare 10 wt % pigment aqueous solution. Thus, obtained was the pigmentdispersion 3 in which an anionically charged self-dispersing carbonblack having a hydrophilic group bonded onto its surface via a phenylgroup, as shown by the following formula, was dispersed.

The pigment dispersion 4 was prepared in the same manner as in Example1.

The ink prepared as above was filled in an ink tank of an ink-jetprinting apparatus described above, and printing was carried out at anejection frequency of 7.2 kHz. The resolution of the printing apparatuswas 360 dpi×720 dpi, and the orifice area had been treated with asilicon ink-repellent. As a result, obtained was a good printed imagehaving large dot size, high optical density, high rub-off resistance,high dot roundness, and no white streaks.

The ink was repelled by the orifice area very well. Thus, high ejectionperformance was sustained. After 1×10⁸ pulses, ejection clogging orirregular ejection was not observed.

Comparative Example 1

As a comparative example to the above Examples 1 to 3, an ink of thefollowing composition was prepared. The pigment dispersion 4 wasprepared in the same manner as in Example 1.

Pigment dispersion 4 50 parts  Ethylene glycol 8 parts Glycerol 5 partsIsopropyl alcohol 4 parts Water balance

The ink prepared as above was filled in an ink tank of an ink-jetprinting apparatus described above, and printing was carried out at anejection frequency of 7.2 kHz. The resolution of the printing apparatuswas 360 dpi×720 dpi, and the orifice area had been treated with asilicon ink-repellent As a result, obtained was a printed image havingsmall dot size, low rub-off resistance, poor dot roundness, and whitestreaks. The ink was poorly repelled by the orifice area. Thus, after1×10⁸ pulses, ejection clogging or irregular ejection was conspicuous.

1. An ink comprising a first pigment, a second pigment, a polymerdispersant and an aqueous medium; the first pigment being aself-dispersing pigment having on its surface at least one anionic groupbonded to the surface directly or via an atomic group, the secondpigment being dispersed in the aqueous medium by the polymer dispersant,and the polymer dispersant being a copolymer of a benzylmethacrylatemonomer unit and a methacrylic acid monomer unit.
 2. The ink accordingto claim 1, wherein the benzylmethacrylate monomer unit and themethacrylic acid monomer unit are present in a molar ratio of 1:1. 3.The ink according to claim 1, wherein the second pigment has a BETspecific surface area of 300 m²/g or less, and a DBP oil absorption of150 mg/100 g or less.
 4. The ink according to claim 1, wherein theanionic group bonded to the first pigment is selected from the groupconsisting of —COOM, —SO₃M, —PO₃HM and —PO₃M₂, —SO₂NH₂, —SO₂NHCOR,wherein M represents a hydrogen atom, alkaline metal, ammonium ororganic ammonium, R represents a linear or branched alkyl group having 1to 12 carbon atoms, a substituted or unsubstituted phenyl group, or asubstituted or unsubstituted naphthyl group.
 5. The ink according toclaim 1, wherein the atomic group is selected from the group consistingof substituted or substituted alkylene of 1-12 carbon atoms, substitutedor unsubstituted phenylene, and substituted or unsubstitutednaphthylene.
 6. The ink according to claim 1, wherein the polymerdispersant has a weight average molecular weight of 2000-30000.
 7. Theink according to clam 1, wherein the polymer dispersant has an acidvalue of 100-500.
 8. The ink according to claim 1, wherein the polymerdispersant is contained in an amount of 10-200% by weight of the secondpigment.
 9. The ink according to claim 1, wherein the second pigment isdispersed in the ink by adsorbing the polymer dispersant on its surface.10. The ink according to claim 1, wherein the first pigment is containedin a ratio to the second pigment of 95/5 to 50/50.
 11. The ink accordingto claim 1, wherein the ink contains a dye.
 12. The ink according toclaim 11, wherein the dye is an anionic dye.
 13. The ink according toclaim 12, wherein the anionic dye is selected from the group consistingof an acidic dye, a direct dye, and a reactive dye.
 14. The inkaccording to claim 1, wherein the ink is for ink-jet printing.
 15. Anink-jet printing method comprising the steps of: ejecting an ink to aprinting medium by an ink-jet process, and forming an image on theprinting medium, wherein the ink is an ink according to claim
 14. 16. Anink-jet printing apparatus comprising an ink-storing part containing anink according to claim 14, and an ink-jet head to eject the ink.
 17. Theink-jet printing apparatus according to claim 16, wherein at least partof an orifice area of the ink-jet head is water repellent.
 18. Theink-jet printing apparatus according to claim 17, wherein at least partof an orifice area of the ink-jet head is covered with a water repellentsilicon film.
 19. The ink-jet printing apparatus according to claim 16,wherein at least part of an orifice area of the ink-jet head is coveredwith a water repellent silicon film.
 20. A printing unit comprising anink-storing part containing an ink according to claim 14, and an ink-jethead to eject the ink.
 21. The printing unit according to claim 20,wherein at least part of an orifice area of the ink-jet head is waterrepellent.
 22. The printing unit according to claim 21, wherein at leastpart of an orifice area of the ink-jet head is covered with a waterrepellent silicon film.
 23. The printing unit according to claim 20,wherein at least part of an orifice area of the ink-jet head is coveredwith a water repellent silicon film.
 24. An ink cartridge comprising anink-storing part containing an ink according to claim 14, wherein theink cartridge is removable and is attached to an ink-jet head thatejects the ink, with the ink cartridge supplying the ink to the ink-jethead when attached thereto.
 25. The ink cartridge according to claim 24,wherein at least part of an orifice area of the ink-jet head is waterrepellent.
 26. An ink cartridge comprising an ink-storing partcontaining an ink according to claim
 1. 27. An ink-jet printing methodcomprising a step of: applying a first ink and a second ink on aprinting medium in such a manner that the first and the second inks comein contact in a liquid state on the printing medium, wherein the firstink contains a self-dispersing pigment having on its surface at leastone anionic group bonded to the surface, directly or via an atomicgroup, the second ink contains an anionic or nonionic polymer dispersantor both and a pigment which is dispersible with the aid of the polymerdispersant in an aqueous medium, the pigment in the second ink has a BETspecific surface area not higher than 300 m²/g, and a DBP oil absorptionnot higher than 150 ml/100 g, and the polymer dispersant is a copolymerof a benzylmethacrylate monomer unit and a methacrylic acid monomerunit.
 28. An ink-jet printing method comprising a step of: applying afirst ink and a second ink on a printing medium in such a manner thatthe first and the second inks come in contact in a liquid state on theprinting medium, wherein the first ink contains a self-dispersingpigment having on its surface at least one anionic group bonded to thesurface, directly or via an atomic group, the second ink contains ananionic or nonionic polymer dispersant or both and a pigment which isdispersible with the aid of the polymer dispersant in an aqueous medium,and the polymer dispersant is a copolymer of a benzylmethacrylatemonomer unit and a methacrylic acid monomer unit.
 29. An ink setcomprising a first ink and a second ink, wherein the first ink containsa self-dispersing pigment having on its surface at least one anionicgroup bonded to the surface, directly or via an atomic group, the secondink contains an anionic or nonionic polymer dispersant or both and apigment which is dispersible with the aid of the polymer dispersant inan aqueous medium, the pigment in the second ink has a BET specificsurface area not higher than 300 m²/g, and a DBP oil absorption nothigher than 150 ml/100 g, and the polymer dispersant is a copolymer of abenzylmethacrylate monomer unit and a methacrylic acid monomer unit. 30.An ink set comprising a first ink and a second ink, wherein the firstink contains a self-dispersing pigment having on its surface at leastone anionic group bonded to the surface, directly or via an atomicgroup, the second ink contains an anionic or nonionic polymer dispersantor both and a pigment which is dispersible with the aid of the polymerdispersant in an aqueous medium, and the polymer dispersant is acopolymer of a benzylmethacrylate monomer unit and a methacrylic acidmonomer unit.
 31. An ink-jet printing apparatus comprising anink-storing part separately containing a first ink and a second ink, andan ink-jet head to eject the first ink and the second ink, wherein thefirst ink contains a self-dispersing pigment having on its surface atleast one anionic group bonded to the surface, directly or via an atomicgroup, the second ink contains an anionic or nonionic polymer dispersantor both and a pigment which is dispersible with the aid of the polymerdispersant in an aqueous medium, the pigment in the second ink has a BETspecific surface area not higher than 300 m²/g, and a DBP oil absorptionnot higher than 150 ml/100 g, and the polymer dispersant is a copolymerof a benzylmethacrylate monomer unit and a methacrylic acid monomerunit.
 32. An ink-jet printing apparatus comprising an ink-storing partseparately containing a first ink and a second ink, and an ink-jet headto eject the first ink and the second ink, wherein the first inkcontains a self-dispersing pigment having on its surface at least oneanionic group bonded to the surface, directly or via an atomic group,the second ink contains an anionic or nonionic polymer dispersant orboth and a pigment which is dispersible with the aid of the polymerdispersant in an aqueous medium, the polymer dispersant is a copolymerof a benzylmethacrylate monomer unit and a methacrylic acid monomerunit.